1. Field of the Invention
The present invention relates to an optical space monitoring type rain sensor.
2. Description of the Prior Art
FIG. 1 illustrates a structural example of a conventional space monitoring type rain sensor. This sensor is mounted, for example, on a motor vehicle, and is used with a so-called automatic windshield wiper drive control system for automatically regulating the intervals of wiping operation depending on the amount of sensed precipitation. A transmitter 1 and a detector 2 are disposed opposed to each other, and they are installed so that both the optical axes thereof coincide with each other. When a direct light beam L1 passing from the transmitter 1 to the detector 2 is cut off or interrupted by an obstacle 4 such as a raindrop such occurrence is detected as a drop of the level of reception light output (amplitude modulation) as shown in FIG. 2 (2). FIG. 2 (1) shows an output level when the pulse-modulated direct light L1 is received directly, i.e. without interruption.
Since the amplitude modulation amount .DELTA.Vl in FIG. 2 (2) is proportional to the shielding area of the obstacle 4, when .DELTA.Vl is large, it indicates a large raindrop, and when small it indicates a small raindrop.
However, since the light beam from the transmitter 1 has a certain spread, there is also other light L2 directed toward a casing 3. Accordingly, when a reflected light beam L3 of L2 enters the detector 2, if there is the same obstacle 4, the amplitude modulation amount .DELTA.V2 becomes smaller than the above .DELTA.Vl, as shown in FIG. 2 (3), and thus there will occur an error in detention, i.e. .DELTA.V2 indicates less water than actually is present may occur. A similar phenomena may also occur, when the optical axis of the transmitter 1 becomes deviated.
Hitherto, in order to reduce this problem, as shown in FIG. 3, formation of the reflected light L3 into the detector 1 is prevented by increasing the height H of installation of the transmitter 1 and detector 2, or roughening the surface of the casing (surface embossing, undulations).
In the method shown in FIG. 3, however, the outer size of the sensor is increased, mounting is restricted and appearance may not be always favorable. Furthermore, by roughening the surface of the casing 3, wax or dust may deposit, the surface may be easily made to appear dirty, and this is not desirable for appearance.